To repeat, a chemical reaction in which substances react together to produce resultants and the resultants in turn react with one another to produce the original substances is known as reversible chemical reaction. For example, decomposition followed by combination reactions of calcium carbonate.
When we heat calcium carbonate (CaCO3) in a closed vessel the evolved carbon dioxide will not go out; it reacts with another product calcium oxide (CaO) to give the calcium carbonate (CaCO3) in reverse. The direction of reaction is affected by like reaction temperature,concentration of reactants, stability of products,etc
When the decomposition reaction is carried out in an open condition, evolved carbon dioxide gas escapes. Therefore, uniting of CO2 gas with calcium oxide is not observed resulting in an irreversible reaction.
Reversible reactions do not proceed to completion in any one direction. Usually the rate of the reverse reaction is slower than that of the forward reaction.